Modern cellular communication networks are vertically structured into individual cell layers. Different cell layers provide wireless network access services via different Radio Access Technology (RAT) types or different configurations of a particular RAT type. For this reason, vertically structured communication networks are also referred to as being heterogeneous.
In heterogeneous network deployments the cell layers typically differ with respect to the coverage area of their individual cells (or, which is equivalent, with respect to the nominal transmit power of their Base Stations, or BSs). In the order of decreasing coverage, macro cells, micro cells, pico cells and femto cells may be defined.
A macro cell provides the largest coverage area at a nominal BS transmit power of some tens of watts. Micro cells are often intended to locally increase network access capacities of macro cells in densely populated areas (e.g., within urban or suburban macro cells). As such, micro cells provide coverage over distances of typically between 300 m and 1000 m from a micro cell BS, which has a nominal transmit power of a few watts. Pico cells provide a more localised coverage than micro cells (at a lower nominal transmit power) and are typically found within buildings such as airport terminals, train stations and shopping centres with insufficient macro or micro cell coverage. In a similar manner, femto cell coverage is even smaller than pico cell coverage at a still lower nominal transmit power.
Today, wireless network access services are mainly provided by Third Generation (3G) communication networks deployed in the form of macro cells. Due to transmit power limitations in mobile terminals, the need for higher throughputs in Fourth Generation (4G) communication networks, especially near the cell edge, combined with the constraint on the uplink (UL) power budget necessitates the introduction of smaller cells such as pico cells.
4G pico cells can utilize different carrier frequencies but can also be overlaid on the same carrier frequencies as used by 3G macro cells. As a consequence, new interference scenarios occur in the resulting heterogeneous networks due to the associated transmit power imbalances and cell association mechanisms. A cell association mechanism defines the BS responsible for a particular mobile terminal (i.e., the serving BS of this mobile terminal). The current cell association mechanisms are mainly based on Reference Signal Received Power (RSRP) measurements performed by the mobile terminal. The RSRP also depends on the transmit power of the serving BS.
In case of a heterogeneous network deployment with macro and pico cell layers as exemplarily shown in FIG. 1A, the RSRP-based cell association and the transmit power imbalance between the two cell layers lead to the following critical situation in the UL. A mobile terminal 8 in the coverage area of both a macro cell 10 and a pico cell 12 (that is located within the macro cell 10) may measure a higher RSRP from the macro BS 14, although it is located closer to the pico BS 16 with lower nominal transmit power. That is, its pathloss to the pico BS 16 is smaller than the pathloss to the macro BS 14. As illustrated in FIG. 1A, this mobile terminal 8 will be served by the macro BS 14 according to the current RSRP-based cell association mechanism. However, to compensate the large pathloss to its serving macro BS 14, the mobile terminal 8 will use a high transmit power. Therefore, signals of this mobile terminal will strongly interfere with signals of mobile terminals associated with the pico BS 16.
One way to avoid this interference scenario is to extend the coverage area of pico BSs, for example by using a cell association mechanism that is based on pathloss. In that case, however, strong downlink (DL) interference will be experienced by mobile terminals served in pico cells and located at the border between the coverage areas of macro and pico cells. This DL interference is caused by the fact that the DL transmit power of the macro BS will remain much higher than the DL transmit power of the pico BS.
Similar or reverse interference problems exist in other deployment scenarios of a heterogeneous communication network. Moreover, mobile terminals served by macro BSs may also be interfered by pico BSs, in particular in DL if access to the pico BS is limited to a group of particular subscribers.